The application of the basic dual wavelength principle to detect hemoglobin and myoglobin changes in tissue began with the work of G. A. Millikan in his studies of the cat soleus muscle, and the work of Millikan and Pappenheimer who detected hemoglobin deoxygenation in the human ear lobe. Multiwavelength instruments have been developed; these instruments use either a multiwavelength laser diode light source or a time shared filter technique, in which high precision is sought through various algorithms which deconvolute background signals, oxidized and reduced cytochrome signals, and oxy- and deoxyhemoglobin signals. Such instruments are oxy-complex and often have difficulty obtaining light sources with wavelengths appropriate to the algorithms that have been developed, or they have such low light levels that photon counting is necessary. They are generally in the price range of $80,000 and have produced much experimental data in the literature on neonates and adults. The basic problem of such methods is that the optical pathlength is not known ab initio but is calculated by reference to animal models where the hemoglobin can be removed and cytochrome directly studied. Transferability of such data from the animal model to the human is one difficulty that had to be overcome prior to the invention of time-resolved spectroscopy, where the pathlength is measured directly. See U.S. patent application Ser. No. 266,166, filed Nov. 2, 1988, "Optical Coupling System for Use in Monitoring Oxygenation State Within Living Tissue," fully referenced above.
Continuous wave spectroscopy (CWS) of tissue hemoglobin has the demonstrated advantages of great simplicity and sensitivity, as well as affording an "early warning" of tissue hypoxia. The application of picosecond-pulse time-resolved spectroscopy (TRS) to tissue in order to determine optical pathlengths, quantify the changes in hemoglobin concentration, and determine the actual concentration values of hemoglobin and cytochrome has great applicability to clinical studies of tissue hypoxia. Moreover, time-resolved spectroscopy used in conjunction with continuous light spectrophotometry offers a means of calibrating the optical pathlength which photons travel as they migrate through tissue. While trend indication can be of great value in many situations, the capability to quantify hemoglobin concentration for both continuous and pulsed light techniques greatly extends their applicability to clinical studies. See U.S. patent application Ser. No. 266,166, filed Nov. 2, 1988, "Optical Coupling System for Use in Monitoring Oxygenation State Within Living Tissue"; and U.S. application Ser. No. 287,847, filed Dec. 21, 1988, "Methods and Apparatus For Determining the Concentration of a Tissue Pigment Of Known Absorbance, In Vivo, Using the Decay Characteristics of Scattered Electromagnetic Radiation", both of which are fully referenced above.